This invention relates to a cylindrically shaped cover for rollers in the dampening apparatus of an offset printing machine.
An offset printing machine possesses a dampening apparatus for moisturizing a printing surface of the plate cylinder.
A commonly used dampening apparatus is composed of several rollers, one of which is covered with the cylindrically shaped cover formed of hydrophilic fibers.
This kind of roller which is called a dampening roller, always stores much water on its surface.
The inventor of this invention has invented covers of the so-called molleton type for this purpose which have been disclosed in Japanese Patent Publication Numbers 47-29650, 47-29651 and 50-33448.
The molletons of the type referred to are formed of high water shrinkable rayon fibers or polyvinyl alcohol fibers so that upon shrinking when wet, they can cause the cover to adhere firmly on the roller body.
However, when it is desired to speed up the printing operation, the an intermittent water supply system of the kind in which water is intermittently supplied to the dampening roller is not suitable. For this reason a continuous water supply system in which water is continuously supplied to the dampening roller should be employed in the high speed offset printing machine.
In the continuous water supply system, the amount of water supplied is controlled by applying a doctor roller, which operates as a doctor knife, to the surface of a pickup roller for picking up water from a water pot, and then by controlling the thickness of the water film on the surface of the pickup roller.
The molleton roller, that is, the dampening roller covered with molleton, is unsuitable for the continuous water supply system because of the following difficulties.
That is, since the molleton is a porous material composed of fibers and has a very large water-holding capacity, it takes a long time for the molleton to release to the plate cylinder all of the water stored in itself, so that it is impossible to decrease immediately the amount of water on the printing surface of the plate cylinder, even though the doctor roller is driven to decrease it.
On the other hand, since it takes a long time for the molleton to absorb and store enough water to adquately supply the plate cylinder, it is impossible to increase immediately the amount of water on the printing surface of the plate cylinder, even though the doctor roller is driven to increase the amount of water being supplied.
Therefore, instead of the molleton roller, a rubber roller of which the surface is composed of rubber is used for the dampening apparatus of the continuous water supply system.
However, since the rubber roller has much poorer hydroscopic and hydrophilic qualities than the molleton roller, uneven spots called rain marks tend to appear, as water tracks along in a circumferential direction, on the water film of its surface due to the centrifugal force and the surface tension of water when the roller is rotated, and they in turn are transferred and make faults called water ghosts on the printed articles.
To solve this problem, alcohol may be added to water to reduce its surface tension. However, this solution is not desirable because the alcohol vapors are injurious to the workers' health.
Furthermore, because the rubber roller has poor hydrophilic qualities, tacky spots of ink tend to stick on its surface, which causes the uneven spots on the surface of the rubber roller and then the irregularity in the amount of water supplied to the plate cylinder, and as a result, it becomes difficult to stably produce the printed articles with high quality.